drm/encoder: add functionality to register encoders to a global list

Commit-Message

This allows standalone encoders to be registered and found again
through their devicetree node when going through their connection graph.
Setting the of_node property is of course still optional, as it is not
necessary in most cases to lookup encoders that are part of a bigger
component.
Signed-off-by: Heiko Stuebner <heiko@...>